Eating Garlic and Onions May Help Prevent Colorectal Cancer—But There’s a Catch

Garlic and onions are a healthy cook’s best friend because they bring major flavor to any dish without adding extra salt or fat—but there may be an even more compelling reason to love these alliums.

Previous research has shown that red onions have cancer cell-busting compounds that could help ward off various forms of the disease, but scientists wanted to see if all alliums could help prevent colorectal cancer.

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Colorectal cancer is the third most common cancer for both men and women in the U.S, and it’s also the third most deadly, according to the American Cancer Society. Researchers are already aware of certain food-related risks that can contribute to the disease—such as eating red meat, smoking, and drinking excessively—but they wanted to see if there were any foods (allium vegetables, in particular) that could help prevent it.

Researchers from the First Hospital of China Medical University followed 833 adults with colorectal cancer, and 833 control participants without it, who were around the same age and lived in the same area for just over two years. They recently published their results in the Asia-Pacific Journal of Clinical Oncology, and they were pretty shocking to say the least.

The study found that adults who consumed the highest number of allium vegetables—specifically onions, spring onions, garlic, garlic stalks, and leeks—were 79 percent less likely to develop colorectal cancer than those who consumed these vegetables at the lowest levels. Interestingly enough, these findings were consistent in both men and women (a previous study found a weak protective effort in women, and a slight increase in risk in men who ate alliums.)

The authors concluded that “consumption of allium vegetables is associated with a reduced risk of CRC, regardless of colonic tumor subsite, with the exception of garlic intake in distal colon cancer.”

So, just how many onions do you have to eat to reap the benefits? The study’s authors say eating 16.02 kg of fresh alliums each year (that’s 35 pounds—or just about 1.5 ounces a day, if you’re trying to do the math) could reduce your colorectal cancer risk. But they’re also quick to note that the cooking method matters, because it could “affect the bioactivity of the vegetables.” The study says that “slicing and crushing fresh garlic are important for the formation of various lipid- and water-soluble organosulfur compounds, while boiling onions lead to about 30% loss of beneficial substance.”